Skeletal system

• Functions of the skeletal system

  • Support
  • Protection
  • Movement
  • Storage
  • Blood cell production - hematopoiesis
• Classification of bones by location

  • Axial skeleton
  • Appendicular skeleton
• Classification of bones by formation

  • Membrane/Dermal bone (formed by direct ossification in connective tissue)
  • Endochondral or Cartilage bone (formed by the replacement of pre-existing cartilage)
• Classification of bones by tissue

  • Spongy/Cancellous bone tissue
  • Compact bone tissue
• Classification of bones by shape

  • Long bones (extremities)
  • Short bones (wrist & ankle)
  • Flat bones (skull, ribs, scapula, & sternum)
  • Irregular bones (vertebra & hip bone)
  • Sesamoid bone (patella)
• Bone markings: depressions & openings


  • Sites of attachment for muscles, ligaments and tendons
  • Serve as joint surfaces
  • Channels for blood vessels and nerves
• Meatus


  Canal-like passage
• Sinus


  Cavity within a bone
• Fossa


  Shallow, basinlike depression
• Fissure


  Narrow, slitlike opening
• Foramen


  Round or oval opening through a bone
• Parts of a long bone

  • Diaphysis/Shaft
  • Epiphysis
• Diaphysis/Shaft


  • Composed of compact bone
  • Covered by periosteum
  • Contains yellow marrow/medullary cavity (adipose tissue)
  • Contains red marrow (hematopoiesis)
• Epiphysis


  • End of long bone
  • Covered by articular cartilage (decreases friction at joint surfaces)
  • Contains epiphyseal plate (causes lengthwise growth of long bone)
  • Contains epiphyseal line (remnant of epiphyseal plate)
• Haversian system


  • Functional unit of bone cell
  • Haversian canal/central canal
  • Lamellae
  • Canaliculi
  • Lacuna
  • Bone cells (osteocytes, osteoblasts & osteoclasts)
• Bone matrix


  • Composed of collagen fibers (about 90% of the organic substance) & ground substance
  • Hardness due to inorganic salts (hydroxyapatite; about 75% of the dry weight of bone)
• Bone cells


  • Osteoprogenitor cells (or stem cells of bone)
  • Osteoblasts (or bone forming cells)
  • Osteocytes
  • Osteoclasts
• Osteoprogenitor cells


  • Located in the periosteum and endosteum
  • Difficult to distinguish from surrounding connective tissue cells
  • Differentiate into osteoblasts
• Osteoblasts


  • May form a low columnar "epitheloid layer" at sites of bone deposition
  • Contain plenty of rough endoplasmatic reticulum (collagen synthesis) and a large Golgi apparatus
  • Become trapped in the forming bone and differentiate into osteocytes
• Osteocytes


  • Contain less endoplasmatic reticulum and are somewhat smaller than osteoblasts
• Osteoclasts


  • Very large (up to 100 µm), multi-nucleated (about 5-10 visible in a histological section, but up to 50 in the actual cell) bone-resorbing cells
  • Arise by the fusion of monocytes (macrophage precursors in the blood) or macrophages
  • Attach themselves to the bone matrix and form a tight seal at the rim of the attachment site
• Intramembranous ossification


  1. Membrane layers of primitive connective tissue appear at sites of future bones

  2. Primitive connective tissue cells become arranged around the blood vessels in these layers

  3. Connective tissue cells differentiate into osteoblasts, which form spongy bone

  4. Osteoblasts become osteocytes when they are completely surrounded by bony matrix

  5. Connective tissue on the surface of each developing structure forms a periosteum

  6. Osteoblasts on the inside of the periosteum form compact bone over the spongy bone
• Endochondral ossification


  1. Masses of hyaline cartilage form models of future bones

  2. Cartilage tissue breaks down and disappears

  3. Blood vessels and differentiating osteoblasts from the periosteum invade the disintegrating tissue

  4. Osteoblasts form spongy bone in the space occupied by cartilage

  5. Compact bone is deposited around the spongy bone by osteoblasts beneath the periosteum
• Types of bone fractures

  • Nondisplaced
  • Displaced
  • Complete
  • Incomplete
  • Linear
  • Transverse
  • Compound (open)
  • Simple (closed)
• Common types of bone fractures

  • Comminuted
  • Spiral
  • Depressed
  • Compression
  • Epiphyseal
  • Greenstick
• Osteomalacia


  • Bones are inadequately mineralized causing softened, weakened bones
  • Main symptom is pain when weight is put on the affected bone
  • Caused by insufficient calcium in the diet, or by vitamin D deficiency
• Rickets


  • Bones of children are inadequately mineralized causing softened, weakened bones
  • Bowed legs and deformities of the pelvis, skull, and rib cage are common
  • Caused by insufficient calcium in the diet, or by vitamin D deficiency
• Osteoporosis


  • Group of diseases in which bone reabsorption outpaces bone deposit
  • Spongy bone of the spine is most vulnerable
  • Occurs most often in postmenopausal women
  • Bones become so fragile that sneezing or stepping off a curb can cause fractures
• Osteoporosis treatments


  • Calcium and vitamin D supplements
  • Increased weight-bearing exercise
  • Hormone (estrogen) replacement therapy (HRT) slows bone loss
  • Natural progesterone cream prompts new bone growth
  • Statins increase bone mineral density
• Rickets


  Bones of children are inadequately mineralized causing softened, weakened bones. Bowed legs and deformities of the pelvis, skull, and rib cage are common. Caused by insufficient calcium in the diet, or by vitamin D deficiency
• Osteoporosis


  Group of diseases in which bone reabsorption outpaces bone deposit. Spongy bone of the spine is most vulnerable. Occurs most often in postmenopausal women. Bones become so fragile that sneezing or stepping off a curb can cause fractures
• Osteoporosis: Treatment


  1. Calcium and vitamin D supplements

  2. Increased weight-bearing exercise

  3. Hormone (estrogen) replacement therapy (HRT) slows bone loss

  4. Natural progesterone cream prompts new bone growth

  5. Statins increase bone mineral density
• Paget's Disease


  Characterized by excessive bone formation and breakdown. Pagetic bone with an excessively high ratio of woven to compact bone is formed. Pagetic bone, along with reduced mineralization, causes spotty weakening of bone. Osteoclast activity wanes, but osteoblast activity continues to work
• Paget's Disease


  • Usually localized in the spine, pelvis, femur, and skull
  • Unknown cause (possibly viral)
  • Treatment includes the drugs Didronate and Fosamax
• Developmental Aspects of Bones

  By age 25, nearly all bones are completely ossified. In old age, bone resorption predominates. A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life
• Joints (Articulations)


  Weakest parts of the skeleton. Articulation – site where two or more bones meet. Functions of joints: Give the skeleton mobility, Hold the skeleton together
• Structural classification of joints

  • Fibrous
  • Cartilaginous
  • Synovial
• Functional classification of joints

  • Synarthroses – immovable
  • Amphiarthroses – slightly movable
  • Diarthroses – freely movable
• Fibrous Structural Joints


  The bones are joined by fibrous tissues. There is no joint cavity. Most are immovable. There are three types – sutures, syndesmoses, and gomphoses
• Fibrous Structural Joints: Sutures


  Occur between the bones of the skull. Comprised of interlocking junctions completely filled with connective tissue fibers. Bind bones tightly together, but allow for growth during youth. In middle age, skull bones fuse and are called synostoses